Conventionally, there are two primary methods employed to back-grind very thin semiconductor wafers. A first method involves laminating a relatively thin, flexible tape to the device side of the wafer prior to back-grinding. In some cases, a layer of photoresist is applied to the device side of the wafer prior to the application of the tape. The wafer is then ground and stress is relieved by chemical etching or chemo-mechanical polishing. The tape is removed by peeling, and the photoresist (if used) is removed by immersion in a hot acid solution or solvent. The thinned wafer is then transferred to a dicing frame (a sheet of adhesive held taught by a square, rigid frame) for die singulation (dicing).
This first method may be adequate for processing where the final wafer thickness is greater than or equal to 300 μm. However, as the final thickness of the ground wafer is decreased, especially when the target reaches 150 μm or less, this method becomes problematic for several reasons. First, the combination of photoresist and tape does not provide sufficient mechanical strength/stiffness to adequately support the thinned wafer as it is removed from the grinder chuck and moved to subsequent processing stations. The insufficient thinned wafer support can result in increased propensity for the wafer to break apart. This problem grows more acute as the diameter of the wafer increases.
A second problem with this method is due to the fact that the combination of photoresist and tape is relatively soft and yielding. The depth of damage induced into the silicon wafer by the grinding process is a function of the stiffness and rigidity of the grinding system (grinding head, spindle, wheel, and chuck) and of the substrate (wafer) being ground: the greater the degree of stiffness of the substrate, the less the depth of grind damage, and vice-versa. Thus, the relatively soft and non-rigid character of the photoresist/tape combination induces a degree of chatter in the grind wheel which limits the final wafer thickness that can be achieved because the wafer can fall apart if the grind damage propagates all the way from the ground surface to the opposing surface of the wafer.
Further, in the case where no photoresist is used, problems can result from removal of the tape. Mechanical peeling of the tape can directly damage delicate device features. Further, stress induced in the overall wafer by the mechanical peeling can lead to warpage and/or curling of the wafer during subsequent processing and handling. The warpage and/or curling may result in wafer breakage or related problems.
In the case where a photoresist coating is used under the back-grinding tape, the removal of the photoresist poses a problem. Photoresist is typically removed with hot acid solutions or organic solvents. Acid solutions and organic solvents are both undesirable with respect to worker health and safety, environmental stewardship, and the costs and complications associated with waste management and removal.
The second primary method employed to back-grind wafers is conventionally used when the desired final wafer thickness falls below the threshold possible using the first method described above. This second method involves mounting the wafer to be thinned onto a rigid support structure (commonly made of stainless steel, ceramic, or quartz) through use of wax or other adhesive. This second method may be used to obtain final wafer thicknesses below 150 μm. However, this second method also has several problems.
First, the method requires that the thinned wafer be separated from its support plate prior to mounting the wafer on a dicing frame. The thinned wafer is therefore vulnerable to damage or breakage during the removal operation and subsequent transfer. The likelihood of this problem occurring increases as the thickness of the wafer decreases.
Second, the method is very difficult to automate. Rather, it is a labor-intensive process and highly dependent on the skills of the operator for its success. Consequently, the method offers a low throughput and a correspondingly high production cost. This method, therefore, is not suitable for cost-effective, automated high-volume production of mainstream commercial products.